A snap-fastener of the foregoing type is already known, comprising an insertable body having a shape resembling that of a harpoon, with two resilient tangs extending upon both sides of the end of a central arm and having a natural position in which the free ends thereof are spaced from the arm and wherein they project outwardly beyond the contour of the opening, and wherein further they lie within this contour when they approach the arm.
The body is inserted by means of its end into the opening, the resilient tangs come closer to the central arm as they come into contact with the wall of the opening, and at the end of the insertion operation, when they have passed through the wall opening, they recover to their spread out expanded position, with their free ends disposed opposite to the wall against which they come to bear so as to prevent the removal of the insertable body.
This snap-fastener is generally entirely satisfactory, however, when it is required to reduce the bulk caused by means of the projection of the snap-fastener below the wall and to therefore diminish the volume of the insertable body, the snap-fastener requires a very high insertion force.